Experiments are designed to characterize mechanisms that may induce coronary arterial vasospasm in acute myocardial ischemia. Transient and sustained effects of increases in the extracellular potassium concentration, acidosis, and hypoxia on the tone of isolated canine coronary arteries before and during electric stimulation and on the simultaneous release of norepinephrine from coronary nerves will be characterized. Discharge of potassium from hypoxic smooth muscle cells and its effects on the release of endogenous norepinephrine and on the tone of hypoxic arteries will be examined. Potassium-dependent modulation of adrenergic effects including the phenomenon of beta adrenergic coronary constriction at elevated potassium concentrations will be studied. Accumulation of cyclic AMP and cyclic GMP in response to adrenergic stimuli in the presence of selected potassium concentrations will be measured. Results in isolated arteries will be evaluated using a new preparation designed to monitor and compare in the same heart simultaneous changes in total coronary resistance and in the vasomotion of a large coronary artery.